1. Field of the Invention
This invention relates to a second harmonic wave-generation (SHG) device preferably usable for a blue laser source device or the like.
2. Related Art Statement
A device to generate a blue laser is suggested which is made by forming an optical waveguide having a periodically polarization-inversed structure and in which an infrared wave semiconductor laser is introduced into the optical waveguide (U.S. Pat. No. 4,740,265, JP-A-5-289131, and JP-A-5-173213). For example, JP-A-6-51359 discloses a SHG device in which a polarization inversion layer, an optical waveguide, a dielectric film, and a reflective grating layer are formed and the thickness of the dielectric film is limited to a given value.
Although these techniques require domains to be controlled at a high precision, such high precision domain-controlling is very difficult. An allowable temperature for the phase-matching must be controlled within a range of .+-.0.5.degree. C. Moreover, light damage of the optical waveguide may be recognized with an optical energy of 3 mW or more. Considering these phenomena, it is pointed out that these devices have a problem when used practically.
On the other hand, NGK Insulators, Ltd. suggested in JP-A-8-339002a SHG device having a reduced light damage without need for quasi-phase-matching or controlling the domains at a high precision. In this literature, a so-called single crystal substrate made of lithium potassium niobate or tantarium-substituted lithium potassium niobate is prepared by a micro pull-down process, and an optical waveguide made of the same material as that of the substrate is formed thereon.
This second harmonic wave-generation device was epock-making in that the optical waveguide for the wavelength conversion successfully reduced an optical loss to a large extent, and for the first time offered a possibility of providing a practical device of this type.
However, it became clarified that the second harmonic wave-generation device needed to be improved in the following respect. That is, the fundamental wave and the second harmonic wave are transmitted through the optical waveguide made of the specific single crystal mentioned above in the second harmonic wave-generation device of this type. Therefore, since the fundamental wave largely differs from the second harmonic wave in terms of the wavelength, the second harmonic wave is of a multi-mode if the transmission mode of the fundamental wave is a single mode. Further, it turned out that a design to make both the fundamental wave and the second harmonic wave of the single mode was not practical. If the second harmonic wave is of a multi-mode, phase matching occurred with respect a wavelength as not aimed at. Further, if the fundamental wave is fluctuated, the second harmonic wave is also fluctuated, so that the oscillation may not stably occur.